There are three main multiple-access methods utilized in cellular communication systems. In frequency-division multiple access (FDMA), each carrier frequency represents a channel that provides a cellular-telephone user access to the system. In time-division multiple access (TDMA), each carrier frequency is divided into a number of time slots, and the time slots represent the channels for accessing the system. In CDMA, there are multiple channels assigned to a single carrier frequency as in TDMA, however, a channel is differentiated from other channels of the same frequency by a spreading code imprinted on the user's transmission rather than by a time slot.
In generalized terms, in a receiver of a CDMA system, two digital signals, or bit streams, are combined together to create a third bit stream that is transmitted over a carrier frequency. The first signal contains information, such as the digitized voice of the cellular-telephone user at a data rate of 9600 bits-per-second (bits/s). The second signal is a stream of pseudorandom bits ("chips") at a data rate ("chip rate"), such as 1.23 Megabits/s, which is produced by a PN sequence generator. The second signal contains the "code" that is imprinted on the information by combining the first signal and the second signal. This combination produces a third signal that contains the information of the first signal and has the chip rate and randomness of the second signal. In this example, a single information bit has 128 PN chips combined with it.
The same code applied to the information signal is used by a receiver in the CDMA system to decode the signals received on the carrier frequency, including the third signal as well as other signals sharing the same frequency, and thus extract the information of the first signal.
The Interim Standard IS-95 has been adopted by the Telecommunications Industry Association for implementing CDMA for the North American cellular telephone system. This standard requires a PN sequence generator that can provide a PN sequence of 32,768 chips (2.sup.15) while simultaneously providing the same PN sequence of chips except shifted by an integer multiple of 64 chips.
For a detailed description of a known PN sequence generator for use in a CDMA system, see U.S. Pat. No. 5,228,054 issued to Rueth et al., incorporated herein by reference. The known PN sequence generator utilizes a 15-stage Linear Sequence Shift Register (LSSR) to generate a PN sequence of 32,767 chips (2.sup.15 -1), and a sequence augmenting circuit that inserts one additional PN chip in the PN sequence of chips, to provide an augmented PN sequence of chips having the required length of 32,768 chips as a primary output. The inserting, or augmenting, is performed by inhibiting for one cycle the change in state of the LSSR when a predetermined 15-bit state of the LSSR is detected, (the LSSR has a respective state for each chip), thus the inserted chip is a repeat of the immediately preceding chip.
The known PN sequence generator also includes a mask circuitry and logic circuitry to provide a shifted version of the PN sequence of chips.
To augment the shifted PN sequence of chips, a correction circuit adds an extra chip to the shifted PN sequence of chips at the appropriate point in the shifted PN sequence of chips and prevents an unwanted chip from being added to the shifted sequence of chips when the extra chip is being inserted in the PN sequence of chips. As was necessary in augmenting the PN sequence of chips to produce the primary output, it is necessary to determine the predetermined state of the LSSR in order to augment the shifted PN sequence of chips.
Because the known PN sequence generator uses the predetermined state of the LSSR in augmenting the PN sequence of chips and shifted PN sequence of chips at the appropriate point in the PN sequence, logic circuitry must be provided that decides when to augment the sequences according to the detected predetermined state of the LSSR. Because this logic circuitry is relatively involved, a need exists for a PN sequence generator that is less complicated.